Load balancing of cloned virtual machines

ABSTRACT

A method, a non-transitory computer-readable storage medium, and a computer system for managing the placement of virtual machines in a virtual machine network are disclosed. In an embodiment, a method involves determining if at least one virtual machine in a set of virtual machines supporting a process and running on a first host computer needs to be separated from other virtual machines in the set. If at least one virtual machine needs to be separated, then at least one virtual machine is selected to be separated based on the number of memory pages changed. The selected VM is then separated from the other virtual machines in the set.

BACKGROUND

Cloud computing and virtual machine (VM) networks have greatly improvedthe flexibility of resource allocation to address workloads incomputational centers. The flexibility is born from the ability toutilize a veritable number of physical machines and VM instancesdepending on the demand of the workload. Typically, when a process isstarted in a VM network, a process begins with a low resource demand(e.g., run on one VM). As the workload handled by the process increases,additional VMs can be instantiated to support the process as needed. Forexample, a simple calculation is initially supported by a single VM.When requests for the calculation begin to be made at a rate faster thanthe VM can perform the calculation, a second VM is instantiated toperform the calculation in parallel with the first VM.

In a typical VM network, workload can increase and decrease quickly.Therefore, rapid instantiation of additional VMs is desirable. However,typically, configuration of newly instantiated VMs can be a timeconsuming process and often limits the responsiveness of a VM network toan increase in workload. Alternatively, a new VM (child VM) can beinstantiated by cloning a VM (parent VM) already configured to supportthe process. By cloning a parent VM, the settings and state of theparent VM can be quickly mapped to the child VM without having toconfigure the child VM. Thus, new child VMs can be created with theappropriate settings and state needed by the process.

However, the creation of new child VMs places further demand on theresources utilized by existing VMs, which can negate the benefits ofcloning or result in underperformance by the existing VMs. In order tomanage resource utilization, VMs may need to be migrated or movedthroughout the VM network. For example, if a parent VM and several childVMs instantiated on the same host computer are over-utilizing the memoryavailable in the host computer (e.g., demanding more memory than isavailable), then a subset of VMs may need to be migrated to another hostcomputer.

SUMMARY

In an embodiment, a method for managing the placement of virtualmachines in a virtual machine network is disclosed. The method involvesdetermining if at least one virtual machine in a set of virtual machinessupporting a process and running on a first host computer needs to beseparated from other virtual machines in the set. If at least onevirtual machine needs to be separated, then at least one virtual machineis selected to be separated based on the number of memory pages changed.The selected VM is then separated from the other virtual machines in theset.

In a second embodiment, a non-transitory computer-readable storagemedium is disclosed. The non-transitory computer-readable storage mediumcontains program instructions that, when executed by one or moreprocessors, causes the one or more processors to determine if at leastone virtual machine in a set of virtual machines supporting a processand running on a first host computer needs to be separated from theother virtual machines in the set. If at least one virtual machine inthe set of virtual machines needs to be separated from the set ofvirtual machines, then at least one of the virtual machines from the setof virtual machines is selected based on the number of memory pageschanged, and the virtual machine is separated from the other virtualmachines in the set.

In a third embodiment, a computer system having at least one hostcomputing device, the host computing device including a processor andmemory for running instantiated virtual machines is disclosed. The atleast one host computing device is configured to determine if at leastone virtual machine in a set of virtual machines supporting a processand running on a first host computer needs to be separated from theother virtual machines in the set and, if at least one virtual machinein the set of virtual machines needs to be separated from the set ofvirtual machines, select at least one of the virtual machines from theset of virtual machines based on the number of memory pages changed andseparate the virtual machine from the other virtual machines in the set.

Other aspects and advantages of embodiments of the present inventionwill become apparent from the following detailed description, taken inconjunction with the accompanying drawings, illustrated by way ofexample of the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a virtual machine network.

FIG. 2 is a block diagram of a host computer from the virtual machinenetwork of FIG. 1.

FIG. 3 illustrates a parent virtual machine supporting a process withseveral child VMs cloned from the parent virtual machine to assist insupporting the process.

FIG. 4 is a block diagram illustrating the memory usage by the child VMsof FIG. 3.

FIG. 5 is a flow diagram of a technique for managing the placement ofvirtual machines in a virtual machine network and selecting a virtualmachine to separate in accordance with an embodiment of the invention.

FIG. 6A illustrates separating a VM from other VMs supporting a processby migrating the VM to a second host computer in accordance with anembodiment of the invention.

FIG. 6B illustrates separating a VM from other VMs supporting a processby migrating the other VMs to a second host computer in accordance withan embodiment of the invention.

Throughout the description, similar reference numbers may be used toidentify similar elements.

DETAILED DESCRIPTION

It will be readily understood that the components of the embodiments asgenerally described herein and illustrated in the appended figures couldbe arranged and designed in a wide variety of different configurations.Thus, the following more detailed description of various embodiments, asrepresented in the figures, is not intended to limit the scope of thepresent disclosure, but is merely representative of various embodiments.While the various aspects of the embodiments are presented in drawings,the drawings are not necessarily drawn to scale unless specificallyindicated.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by this detailed description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

Reference throughout this specification to features, advantages, orsimilar language does not imply that all of the features and advantagesthat may be realized with the present invention should be or are in anysingle embodiment of the invention. Rather, language referring to thefeatures and advantages is understood to mean that a specific feature,advantage, or characteristic described in connection with an embodimentis included in at least one embodiment of the present invention. Thus,discussions of the features and advantages, and similar language,throughout this specification may, but do not necessarily, refer to thesame embodiment.

Furthermore, the described features, advantages, and characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. One skilled in the relevant art will recognize, in light ofthe description herein, that the invention can be practiced without oneor more of the specific features or advantages of a particularembodiment. In other instances, additional features and advantages maybe recognized in certain embodiments that may not be present in allembodiments of the invention.

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the indicatedembodiment is included in at least one embodiment of the presentinvention. Thus, the phrases “in one embodiment,” “in an embodiment,”and similar language throughout this specification may, but do notnecessarily, all refer to the same embodiment.

Turning now to FIG. 1, a block diagram of a virtual machine network 100(VM network) is shown. The VM network includes a network 102, clustersC-1, C-2 . . . C-N (where N is a positive integer) of host computers,and a datastore cluster 104. The exact number of host computer clustersincluded in the distributed computer system can be, for example, from afew clusters to tens of clusters or more. The host computers of thedifferent clusters and the datastore cluster are connected to thenetwork. Thus, each of the host computers in the clusters is able toaccess the datastore cluster via the network and may share the resourcesprovided by the datastore cluster with the other host computers.Consequently, any process running on any of the host computers may alsoaccess the datastore cluster via the network.

In the illustrated embodiment, each of the clusters C-1, C-2 . . . C-Nincludes a number of host computers H-1, H-2 . . . H-M (where M is apositive integer) and a cluster management server 110. The number ofhost computers included in each of the clusters can be any number from,for example, one to several hundred or more. In addition, the number ofhost computers included in each of the clusters can vary so thatdifferent clusters can have a different number of host computers. Whileat least some of the host computers may be virtualized, in theembodiment of FIG. 1, the host computers are physical computer systemsthat host or support one or more VMs so that the VMs are executing onthe physical computer systems. The host computers may be servers thatare commonly found in data centers. As an example, the host computersmay be servers installed in one or more server racks. Typically, thehost computers of a cluster are located within the same server rack.

Each of the cluster management servers 110 in the clusters C-1, C-2 . .. C-N operates to monitor and manage the host computers H-1, H-2 . . .H-M in the respective cluster. Each cluster management server may beconfigured to monitor the current configurations of the host computersand the VMs running on the host computers, for example, virtual machines(VMs), in the respective cluster. The monitored configurations mayinclude the hardware configuration of each of the host computers, suchas CPU type and memory size, and/or software configurations of each ofthe host computers, such as operating system (OS) type and installedapplications or software programs. The monitored configurations may alsoinclude VM hosting information, i.e., which VMs are hosted and runningon which host computers. The monitored configurations may also includeVM information. The VM information may include the size of each of theVMs, virtualized hardware configurations for each of the VMs, such asvirtual CPU type and virtual memory size, software configurations foreach of the VMs, such as OS type and installed applications or softwareprograms running on each of the VMs, and virtual storage size for eachof the VMs. The VM information may also include resource parametersettings, such as demand, limit, reservation and share values forvarious resources, e.g., CPU, memory, network bandwidth and storage,which are consumed by the VMs. The demands of the VMs for the consumableresources are determined by the host computers hosting the VMs bymonitoring the current usage of resources by the VMs, e.g., CPUprocessing usage, memory usage, network usage and/or storage usage, andprovided to the respective cluster management server.

In some embodiments, the cluster management servers 110 may beimplemented on separate physical computers. In other embodiments, thecluster management servers may be implemented as software programsrunning on a host computer or on virtual computers running on a hostcomputer (now shown). In an implementation, the cluster managementservers are VMware vCenter™ servers with at least some of the featuresavailable for such servers and each resource management module (RMM) 112is a VMware Distributed Resource Scheduler™, which provides aDistributed Resource Scheduler (DRS) service as is known in the field.

The network 102 can be any type of computer network or a combination ofnetworks that allows communications between devices connected to thenetwork using, for example, a TCP or UDP transport layer. The network102 may include the Internet, a wide area network (WAN), a local areanetwork (LAN), a storage area network (SAN), a fibre channel networkand/or other networks. The network 102 may be configured to supportprotocols suited for communications with storage arrays, such as FibreChannel, Internet Small Computer System Interface (iSCSI), Fibre Channelover Ethernet (FCoE) and HyperSCSI.

The datastore cluster 104 is used to store data for the host computersof the clusters C-1, C-2 . . . C-N, which can be accessed like any othertype of storage device commonly connected to computer systems. In anembodiment, the datastore cluster can be accessed by entities, such asVMs running on the host computers, using any file system, e.g., virtualmachine file system (VMFS) or network file system (NFS). The datastorecluster includes one or more computer data storage devices 116, whichcan be any type of storage devices, such as solid-state devices (SSDs),hard disks or a combination of the two. At least some of these storagedevices may be local storage devices of the host computers, e.g.,locally attached disks or SSDs within the host computers. The storagedevices may operate as components of a network-attached storage (NAS)and/or a storage area network (SAN). The datastore cluster includes astorage management module 118, which manages the operation of thedatastore cluster. In an embodiment, the storage management module is acomputer program executing on one or more computer systems (not shown)of the datastore cluster. The datastore cluster supports multipledatastores DS-1, DS-2 . . . DS-X (where X is a positive integer), whichmay be identified using logical unit numbers (LUNs). In an embodiment,the datastores are virtualized representations of storage facilities.Thus, each datastore may use resources from more than one storage deviceincluded in the datastore cluster. The datastores are used to store dataassociated with the VMs supported by the host computers of the clustersC-1, C-2 . . . C-N. For virtual machines, the datastores may be used asvirtual storage or virtual disks to store files needed by the virtualmachines for operation. One or more datastores may be associated withone or more clusters. In an embodiment, the same datastore may beassociated with more than one cluster.

Turning now to FIG. 2, components of a host computer 200 that isrepresentative of the host computers H-1, H-2 . . . H-M, are shown. InFIG. 2, the physical connections between the various components of thehost computer are not illustrated. In the illustrated embodiment, thehost computer is configured to support a number of VMs 220A, 220B . . .220L (where L is a positive integer). The number of VMs supported by thehost computer can be anywhere from one to more than one hundred. Theexact number of VMs supported by the host computer is limited by thephysical resources of the host computer or other constraints such aslicensing. The VMs share at least some of the hardware resources of thehost computer, which includes system memory 222, one or more processors224, a storage interface 226, and a network interface 228. The systemmemory 222, which may be random access memory (RAM), is the primarymemory of the host computer. The processor 224 can be any type ofprocessor, such as a central processing unit (CPU) commonly found in aserver. The storage interface 226 is an interface that allows the hostcomputer to communicate with the datastore cluster 104 in FIG. 1. As anexample, the storage interface may be a host bus adapter or a networkfile system interface. The network interface 228 is an interface thatallows the host computer to communicate with other devices in thecluster as well as devices connected to the network 102 in FIG. 1. As anexample, the network interface may be a network adapter. The guestoperating system is a master control program of the guest VM and, amongother things, forms a software platform on top of which guestapplications run. In an embodiment, guest applications are individualprograms such as, for example, an email manager, a system logger, oranother program with which a user directly communicates.

In the embodiment of FIG. 2, the VMs 220A, 220B . . . 220L run on top ofa hypervisor 230 that includes a kernel (not shown). In otherembodiments, one or more of the VMs can be nested, i.e., a VM running inanother VM. For example, one of the VMs may be running in a VM, which isalso running in another VM. The hypervisor may run on top of the hostcomputer's operating system or directly on hardware of the hostcomputer. With the support of the hypervisor, the VMs providevirtualized computer systems that give the appearance of being distinctfrom the host computer and from each other.

Similar to any other computer system connected to the network 102 inFIG. 1, the VMs 220A, 220B . . . 220L shown in FIG. 2 are able tocommunicate with other computer systems connected to the network usingthe network interface 228 of the host computer 200. In addition, the VMsare able to access the datastore cluster 104 in FIG. 1 using the storageinterface 226 of FIG. 2 of the host computer.

The host computer 200 also includes a local resource allocation module236 that operates as part of a resource management system, such as adistributed resource scheduler system, to manage resources consumed bythe VMs 220A, 220B . . . 220L. The local resource allocation module ineach host computer cooperatively operates with the local resourceallocation modules in the other host computers of the network computersystem 100 to generate resource allocation settings and perform resourcescheduling, which includes balancing the loads of software processesand/or storage resource scheduling, among the host computers H-1, H-2 .. . H-M of the host computer clusters C-1, C-2 . . . C-N. Although thelocal resource allocation module is illustrated in FIG. 2 as beingseparate from the hypervisor 230, the local resource allocation modulemay be implemented as part of the hypervisor. In some embodiments, thelocal resource allocation module is implemented as software programsrunning on the host computer. However, in other embodiments, the localresource allocation module may be implemented using any combination ofsoftware and hardware.

A VM in a host computer as described above with reference to FIG. 2 canbe configured to support a process for handling a given workload. FIG. 3is a block diagram of a host computer configured to run a set of VMssupporting a process for handling a given workload. As shown in FIG. 3,the host computer includes a parent VM 302, four child VMs 304, andmemory 306 used by the VMs. Typically, when a host computer isconfigured to support a process, the first VM instantiated to supportthe process is referred to as the “parent VM.” In the example of FIG. 3,when the parent VM is instantiated and configured, the configuration isstored in a configuration file 308 that directs the parent VM to accessselect pages 310 of memory in the memory of the host computer or fromanother shared memory source. The parent VM handles the workload until apredefined point, referred to as a “fork point,” is reached and, oncethe fork point is reached, additional VMs, referred to as “child VMs”are cloned from the parent VM (i.e., instantiated with the sameconfiguration file as the parent VM) to assist in supporting the parentVM. For example, as illustrated in FIG. 3, the configuration file of theparent VM is copied to each of the child VMs and the child VMs aredirected to also use pages in the memory of the host computer or theshared memory source. Thus, initially, a parent VM is instantiated tosupport processing a new workload and the configuration file directs theparent VM to utilize pages in the memory of the host computer or theshared memory source. After a parent process performs some initialcomputations on the workload, a fork point is reached and child VMs areinstantiated to run child processes to further modify the results of theinitial computations or to increase the number of VMs processing theworkload. The child VMs are instantiated with the same configurationfile as the parent VM and are directed to use pages of the memory of thehost computer or the shared memory source as well.

Instantiating child VMs to further modify the results of the initialcomputations or to increase the number of VMs supporting a process canproduce significant savings in resource consumption as well as increasedthroughput. However, increasing the number of VMs available to support aprocess is not without cost. Each child VM is instantiated with the sameconfiguration (e.g., same amount of memory) as the parent VM. Thus,among other resources required, the total amount of memory required byVMs supporting a process increases proportional to the number of VMs.For example, the amount of memory needed can be calculated as follows:Total Memory=n*MemSizewhere n equals the number of VMs supporting the process and MemSizeequals the amount of memory required by the parent VM. By instantiatingeach child VM with a fixed amount of memory, the amount of memory neededcan grow very large.

In order to reduce the amount of memory needed, the child VMs can beconfigured to share the memory pages of the parent VM and, if one of thechild VMs needs to write to a page, a page of the shared memory pagescan be copied to dedicated memory for the child VM when the VM needs towrite to the page. FIG. 4 is a block diagram illustrating memory usageby the child VMs of FIG. 3. In the block diagram of FIG. 4, the childVMs 304 each use shared memory 402 and dedicated memory 404. The sharedmemory and dedicated memory are made up of pages 412, 414 of memory asindicated by the indexed slices. When a child VM needs to read a page inthe shared memory, the child VM accesses the page in the shared memory.Alternatively, when the child VM attempts to write to the page, thechild VM creates a copy of the page 412 from the shared memory into thededicated memory for the child VM and writes to the copied page 414 inthe dedicated memory. Thus, rather than creating full copies of all thepages in the shared memory for each child VM, copies of pages from theshared memory can be created as needed for each child VM. For example,in FIG. 4, when child VM 1 needs to write to pages x007 and x011, eachpage is copied into the dedicated memory for VM 1 and child VM 1 writesto the copied pages in its dedicated memory.

By sharing pages of memory, the amount of memory needed by all VMsinstantiated to support the process can be calculated as follows:Total Memory=(n*(1−Δ))*MemSizewhere n equals the number of VMs supporting the process, Δ equals thepercentage of pages shared by the VMs supporting the process, andMemSize equals the amount of memory required by the parent VM. Thus,sharing pages of memory requires a fraction of the memory that isrequired when all of the pages of memory for each VM are copied. Forexample, in FIG. 4, rather than requiring 100 pages of memory (i.e., acomplete copy of the 25 pages of shared memory for each child VM), only35 pages of memory are required (i.e., one copy of the 25 pages ofshared memory, 2 pages of dedicated memory for child VM 1, 1 page ofdedicated memory for child VM 2, and 7 pages of dedicated memory forchild VM 4).

While using shared memory reduces the amount of memory needed by parentand child VMs, in order for a child VM to use the shared pages ofmemory, the child VM must be able to access the same memory as other VMssharing the pages. Typically, VMs running on the same host computer canall access the same memory while VMs stored on different host computersmay not be able to access the same memory. Thus, if additional child VMsneeded to support the parent process are created on, or migrated to, ahost computer that is unable to access the same memory as the parent VM,then memory pages cannot be shared with the additional child VMs.

While VMs on the same host computer as the parent VM or using the sameshared memory source can share memory pages, not all VMs will share thesame amount of memory pages (e.g., one child VM may not require anypages to be copied to its dedicated memory while another child VM canrequire many pages to be copied to its dedicated memory). As a result,some child VMs will require less memory than others. For example, inFIG. 4, child VM 3 does not write to any pages of memory and, therefore,does not require dedicated pages of memory in addition to the pages fromthe shared memory, whereas child VM 4 writes to seven pages of memoryand requires seven pages of dedicated memory in addition to the pagesfrom the shared memory.

If more child VMs are instantiated than can be supported by a singlehost computer, then some child VMs will need to be separated from otherchild VMs. In accordance with an embodiment of the invention, if it isdetermined that a VM needs to be separated from the other VMs, a VM willbe selected based on the number of memory pages that have been changed(e.g., the number of pages copied to the VM's dedicated memory). Forexample, the number of pages copied to the dedicated memory is used asan indicator to determine how to best migrate VMs. In an embodiment, theVM that has changed the highest number of memory pages (e.g., copied thehighest number of pages to its dedicated memory) will be selected andseparated onto a separate host computer. Thus, by selecting a VM thathas changed the highest number of memory pages from a set of VMssupporting a process and separating that VM onto a separate hostcomputer, more memory will be made available to the other VMs supportingthe process than if a VM that has changed fewer memory pages wereselected and separated.

FIG. 5 is a flow diagram of a technique for managing the placement ofVMs in a virtual machine network. At decision point 502, it isdetermined whether a process running in a VM on a first host computerneeds an additional VM to help support the process. In an embodiment,the determination is made by a DRS running in a cluster in which thehost computer is located. If an additional VM is needed (i.e., a childVM), at decision point 504, it is determined whether an additional VMcan be added to the first host computer. If an additional VM can beadded to the first host computer, then, at block 506, an additional VMis instantiated on the first host computer, the VM is added to the setof VMs supporting a process, and the flow returns to decision point 502.In an embodiment, the instantiated VM is a child VM cloned from a parentVM already running on the first host computer (i.e., has the sameconfiguration file as the VM already running on the first hostcomputer). Alternatively, if a VM cannot be added to the first hostcomputer, then, at block 508, the number of memory pages changed by eachVM running on the first host computer is calculated. At block 510, a VMis selected to be separated from the other VMs based on the number ofmemory pages changed. In an embodiment, the VM that has changed thehighest number of memory pages is selected. In another embodiment, atleast one VM that has changed a number of memory pages in excess of apredefined threshold is selected. For example, if a user sets athreshold at 20%, then at least one of several VMs that have changed 20%or more memory pages is selected. Once a VM has been selected, the VM isseparated from the other VMs in the set of VMs supporting the processand the flow returns to decision point 504 to determine if the neededadditional VM can now be added to the first host computer. If theadditional VM still cannot be added, then the steps for selecting a VMto be separated are repeated.

In accordance with an embodiment of the invention, a VM can be separatedfrom the other VMs in many ways. FIG. 6A depicts a first way ofseparating a VM from other VMs. In FIG. 6A, child VM 1 304, child VM 2304, child VM 3 304, and child VM 4 304 run on host computer A 200. Asdescribed above with reference to FIG. 4, child VM 4 has changed thehighest number of memory pages. In accordance with an embodiment of theinvention, child VM 4 is selected and separated from child VM 1, childVM 2, and child VM 3 by being migrated to host computer B 200. FIG. 6Bdepicts a second way of separating a VM from other VMs. In FIG. 6B,child VM 1 304, child VM 2 304, child VM 3 304, and child VM 4 304 runon host computer A 200. As described above with reference to FIG. 4,child VM 4 has changed the highest number of memory pages and,therefore, is selected to be separated from the other VMs in the set ofVMs supporting the process. In accordance with an embodiment of theinvention, child VM 4 is selected and separated from child VM 1, childVM 2, and child VM 3 by migrating child VM 1, child VM 2, and child VM 3to host computer B 200.

Thus, when an additional VM is needed, separating VMs making fewerchanges to memory pages, referred to as having a “low dirtying rate,” isfavored over separating VMs making many changes to memory pages,referred to as having a “high dirtying rate.” By favoring VMs with a lowdirtying rate, less dedicated memory is needed in addition to the sharedmemory, allowing for less memory to be used by the VMs.

Although the operations of the method(s) herein are shown and describedin a particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operations may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be implemented in anintermittent and/or alternating manner.

It should also be noted that at least some of the operations for themethods may be implemented using software instructions stored on acomputer useable storage medium for execution by a computer. As anexample, an embodiment of a computer program product includes a computeruseable storage medium to store a computer readable program that, whenexecuted on a computer, causes the computer to perform operations, asdescribed herein.

Furthermore, embodiments of at least portions of the invention can takethe form of a computer program product accessible from a computer-usableor computer-readable medium providing program code for use by or inconnection with a computer or any instruction execution system. For thepurposes of this description, a computer-usable or computer readablemedium can be any apparatus that can contain, store, communicate,propagate, or transport the program for use by or in connection with theinstruction execution system, apparatus, or device.

The computer-useable or computer-readable medium can be an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system(or apparatus or device), or a propagation medium. Examples of acomputer-readable medium include a semiconductor or solid state memory,magnetic tape, a removable computer diskette, a random access memory(RAM), a read-only memory (ROM), a rigid magnetic disc, and an opticaldisc. Current examples of optical discs include a compact disc with readonly memory (CD-ROM), a compact disc with read/write (CD-R/W), a digitalvideo disc (DVD), and a Blu-ray disc.

In the above description, specific details of various embodiments areprovided. However, some embodiments may be practiced with less than allof these specific details. In other instances, certain methods,procedures, components, structures, and/or functions are described in nomore detail than to enable the various embodiments of the invention, forthe sake of brevity and clarity.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. The scope of theinvention is to be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A method for managing the placement of a set ofvirtual machines in a virtual machine network, the method comprising:determining, by a processor, if instantiated virtual machines in a setof virtual machines supporting a process and running on a first hostcomputer can be supported by the first host computer; and if at leastone instantiated virtual machine cannot be supported by the first hostcomputer, selecting at least one of the virtual machines from the set ofvirtual machines that has copied a number of pages to dedicated memoryin excess of a pre-defined threshold; and separating the at least onevirtual machine from the other virtual machines in the set of virtualmachines.
 2. The method of claim 1, wherein separating the at least onevirtual machine comprises migrating the at least one virtual machine toa second host computer.
 3. The method of claim 1, wherein separating theat least one virtual machine comprises migrating the other virtualmachines in the set of virtual machines to a second host computer andleaving the at least one virtual machine on the first host computer. 4.The method of claim 1, wherein the set of virtual machines supporting aprocess comprises a parent virtual machine and at least one childvirtual machine cloned from the parent virtual machine.
 5. The method ofclaim 1, wherein the at least one virtual machine in the set of virtualmachines is initialized to use pages of shared memory and to copy a pagefrom the shared memory over to dedicated memory when the at least onevirtual machine needs to write to the page.
 6. The method of claim 1,wherein the at least one virtual machine separated from the othervirtual machines in the set of virtual machines has changed the highestnumber of memory pages.
 7. The method of claim 1, wherein the at leastone virtual machine in the set of virtual machines supporting a processand running on a first host computer needs to be migrated when the firsthost computer cannot support all the virtual machines in the set ofvirtual machines.
 8. The method of claim 1, wherein the pre-definedthreshold is defined as a percentage of pages copied from shared memoryto dedicated memory.
 9. A non-transitory computer-readable storagemedium containing program instructions, wherein execution of the programinstructions by one or more processors causes the one or more processorsto perform steps comprising: determining if instantiated virtualmachines in a set of virtual machines supporting a process and runningon a first host computer can be supported by the first host computer; ifat least one instantiated virtual machine cannot be supported by thefirst host computer, selecting at least one of the virtual machines fromthe set of virtual machines that has copied a number of pages todedicated memory in excess of a pre-defined threshold; and separatingthe at least one virtual machine from the other virtual machines in theset of virtual machines.
 10. The non-transitory computer-readablestorage medium of claim 9, wherein separating the at least one virtualmachine comprises migrating the at least one virtual machine to a secondhost computer.
 11. The non-transitory computer-readable storage mediumof claim 9, wherein separating the at least one virtual machinecomprises migrating the other virtual machines in the set of virtualmachines to a second host computer and leaving the at least one virtualmachine on the first host computer.
 12. The non-transitorycomputer-readable storage medium of claim 9, wherein the set of virtualmachines supporting a process comprises a parent virtual machine and atleast one child virtual machine cloned from the parent virtual machine.13. The non-transitory computer-readable storage medium of claim 9,wherein the at least one virtual machine in the set of virtual machinesis initialized to use pages of shared memory and to copy a page from theshared memory over to dedicated memory when the at least one virtualmachine needs to write to the page.
 14. The non-transitorycomputer-readable storage medium of claim 9, wherein the at least onevirtual machine separated from the other virtual machines in the set ofvirtual machines has changed the highest number of memory pages.
 15. Thenon-transitory computer-readable storage medium of claim 9, wherein theat least one virtual machine in the set of virtual machines supporting aprocess and running on a first host computer needs to be migrated whenthe first host computer cannot support all the virtual machines in theset of virtual machines.
 16. The non-transitory computer-readablestorage medium of claim 9, wherein the pre-defined threshold is definedas a percentage of pages copied from shared memory to dedicated memory.17. A computer system comprising: at least one host computing device,the host computing device including a processor and memory for runninginstantiated virtual machines; and the at least one host computingdevice being configured to: determine if instantiated virtual machinesin a set of virtual machines supporting a process and running on a firsthost computer can be supported by the first host computer; and if atleast one instantiated virtual machine in the set of virtual machinescannot be supported by the first host computer, select at least one ofthe virtual machines from the set of virtual machines that has copied anumber of pages to dedicated memory in excess of a pre-definedthreshold; and separate the at least one virtual machine from the othervirtual machines in the set of virtual machines.
 18. The computer systemof claim 17, wherein separating the at least one virtual machinecomprises migrating the at least one virtual machine to a second hostcomputer.
 19. The computer system of claim 17, wherein separating the atleast one virtual machine comprises migrating the other virtual machinesin the set of virtual machines to a second host computer and leaving theat least one virtual machine on the first host computer.
 20. Thecomputer system of claim 17, wherein the set of virtual machinessupporting a process comprises a parent virtual machine and at least onechild virtual machine cloned from the parent virtual machine.
 21. Thecomputer system of claim 17, wherein the at least one virtual machine inthe set of virtual machines is initialized to use pages of shared memoryand to copy a page from the shared memory over to dedicated memory whenthe at least one virtual machine needs to write to the page.
 22. Thecomputer system of claim 17, wherein the at least one virtual machineseparated from the other virtual machines in the set of virtual machineshas changed the highest number of memory pages.
 23. The computer systemof claim 17, wherein the at least one virtual machine in the set ofvirtual machines supporting a process and running on a first hostcomputer needs to be migrated when the first host computer cannotsupport all the virtual machines in the set of virtual machines.
 24. Thecomputer system of claim 17, wherein the pre-defined threshold isdefined as a percentage of pages copied from shared memory to dedicatedmemory.